The present invention relates to improved gripper bushing assemblies for use with injection molds and molding machines. The gripper bushing assemblies of the present invention are designed for fast cycling applications.
The concept of a gripping bushing for selectively gripping or releasing a rotating or sliding shaft is well known in the art. Early versions of these gripping bushings used radial segments that were pressed into engagement with the shaft by springs or tapered sleeves and were released by hydraulic pressure. U.S. Pat. Nos. 2,691,799 to Moeller; 3,420,144 to Berry; and 3,995,534 to Rastetter and German Offenlegungsschrift 35 106 43 to Sitema illustrate this type of gripper bushings.
Another version of early gripper bushings was based on a thin sleeve deflected inwardly by external hydraulic pressure in such a way as to grip the shaft. The hydraulic pressure used with these bushings was high enough to flex the comparatively thin wall of the bushing. U.S. Pat. No. 4,269,229 to Falk and German Patent Application No. 25 179 97 to Kostyrka illustrate this type of gripper bushing.
Many of these early gripper bushing assemblies were used in applications which were slow cycling or slow acting. Typically, they were used as safety brakes for vertical presses, elevators, and the like, or as couplings for transmitting rotating forces on shafts. The service life of such a bushing according to one manufacturer was about 2 million cycles. While this service life is adequate for the above named applications, it is inadequate for molding machine applications in which 2 million cycles represent 10 months of operation on a molding machine running continuously at 5 cycles per minute. Such a relatively short service life is unacceptable in molding machines.
Gripper bushing assemblies have also been applied to the gripping of tiebars of injection molding machines. The earliest known use of such assemblies in injection molding machines is U.S. Pat. No. 3,183,555 to Siegel. In the Siegel apparatus, a deformable sleeve is part of the tiebar itself. The sleeve is deflected outwardly to grip the inside surface of a tiebar bushing held in a moving platen. Thus, the platen could be gripped quickly prior to clamping the mold with the main clamping unit. In order to grip the platen in a variety of stroke positions, the deformable sleeve extended a considerable length along the tiebar. A typical construction of the sleeve assembly is shown in FIG. 3 of the Siegel patent. In this construction, the sleeve engaged the tiebar by a series of interrupted thread portions. An alternative version of the Siegel assembly is shown in FIG. 4 of the Siegel patent. In this embodiment, a deformable bushing in the platen is used to grip the tiebar. The bushing is provided with a thread for engaging a similarly threaded portion of the platen.
French Patent No. 2,174,361 to Guerin and Japanese Patent Document No. 1-49088 to Japan Steel Works illustrates similar gripper bushing assemblies used in molding machines.
Gripper bushing assemblies have found wide acceptance in vertical molding machines and presses. U.S. Pat. No. 3,938,362 to Falk shows a gripper bushing used in a vertical press. A threaded engagement between the deformable sleeve and the outer portion of the bushing is shown in published PCT Application WO 88/07447 to Hauch. A similar arrangement is shown in Japanese Kokai No. 1-245999 to Kohtaki and in Japanese Kokai No. 2-172711 to Japan Steel Works. These references suggest that it is necessary to make the bonding strength between the deformable sleeve and the bush/piston powerful enough to endure a clamping counterforce reaction. Strength is increased by having the deformable sleeve and a clamp ram joined by screws or threads whose entire length is formed around the sleeve. In this way, the stress of the sleeve is presumably dispersed. However, these systems typically suffer from a fretting problem in which the lead threads are deformed or worn at a relatively high rate.
U.S. Pat. No. 4,509,910 to Petersen illustrates still another clamp assembly for use on vertical presses. The Petersen clamp design includes an inner clamp sleeve positioned concentrically about a column to be gripped. It further includes an outer sleeve secured to the crosshead assembly concentrically about the inner clamp sleeve. The inner and outer clamp sleeves are threadably secured together to limit longitudinal movement of one clamp sleeve with respect to the other. The outer clamp sleeve is secured to the crosshead so that it cannot move relative to the crosshead. Hydraulic fluid is introduced into a chamber formed between the opposing threads of the two sleeves in order to deform the inner sleeve against the column to be gripped. The deformable sleeve is preferably made from aluminum.
Petersen states in column 9, lines 31 through 35 of his patent that stress concentration is inherently "slipped out" at the friction interfaces of the column clamping assembly. Petersen states that this prevents progressive failure which is a common problem in long cylinder threaded joints. In other words, Petersen ignores the fretting problem which occurs in the prior art threaded systems.
Tests have indicated that simple (unthreaded) gripping sleeves prematurely wear at one end. This fact was recognized by the Siegel patent and the aforementioned Japanese patent publications. The use of a threaded engagement between the sleeve and the body of the bushing was an attempt to minimize the stress concentrations which occur. While these threads improve the situation, they can not uniformly distribute the stress along the length of the sleeve. In fact, they introduce new disadvantages.
The need for some mechanical interlocking between the deformable sleeve and a housing body in a two component assembly means that a thread must inevitably be used since any other means prevents assembly. One must bear in mind however that the space between the deformable gripper sleeve and the body is a pressure vessel or chamber subjected to very high pressures, typically 8,000 psi, needed to deform the thin sleeve inward to grip the tiebar. Thus, if the sleeve or the body are not each made in one piece, a problem of sealing the chamber to contain the pressure arises. Hence a threaded connection between the sleeve and the body permits assembly and maintains chamber integrity.
There are two disadvantages to the threaded approach. First, while helping transfer the stress from the sleeve to the body more uniformly, it does nothing to more evenly distribute the stress in the sleeve itself. Second, the thread considerably strengthens the inner sleeve by increasing its thickness by virtue of the male portion of the thread. The hoop strength of the sleeve is considerably increased requiring even higher pressures of hydraulic fluid to be used in order to deform the sleeve inwardly and develop sufficient friction to grip the tiebar. This is counterproductive in the design and increases the potential for leaking.
Accordingly, it is a principal object of the present invention to provide an improved gripper bushing construction.
It is a further object of the present invention to provide a gripper bushing construction as above for use in fast cycling applications.
It is yet a further object of the present invention to provide a gripper bushing construction as above for use in connection with injection molds, molding apparatuses and other devices.
These and other objects and advantages will become more apparent from the following description and drawings in which like reference numerals depict like elements.